Arrangement for protecting the electrodes in demountable high vacuum tubes



Jan. 7, 1947. M, GRAF 2,413,760

f ARRANGEMENT FOR PROTEGTING THE ELECTRODES 1N DEMOUNTABLE HIGH VACUUMTUBES Filed Feb. 4, 1945 Patented Jan. 7, 1947 ARRANGEMENT FORyDEMOUNTABLE HIGH ELECTRODES IN PROTECTING THE VACUUM TUBES Max l Graf,Baden, Switzerland, assigner to Patelhold Patentverwertungs ik Elektro'-Holding` A.Gf, Glarus,.'Switzcrland Application February 4, 1943, SerialNo. 474,754 In Switzerland December 10, 1941:

4 Claims.

It is known that in high vacuum tubes the vacuum can deteriorate andthis may cause ignitions with the result that a cathode spot forms onthe cathode or on a grid which may resultl in the electrodes beingdestroyed. In order to avoid this risk of destruction lt has been usualto employ excess-current relays on the assumption that such ignitionsare identical with short-circuits and therefore result 1n high excesscurrents. This is, however, not correct. Under certain circumstances,for instance when the anode voltage is reduced or when there is acircuit with large external resistance, the current does not increaseunduly when an ignition occurs, although a cathode spot is formed. Theelectrodes are endangered, however, because the excess-currentprotection does not operate.

According to the present invention therefore the condition of the vacuumin the tube and not the magnitude of the anode current is taken as thecriterion for the operation of the protective device.

Demountable tubes have already been equipped with high vacuum measuringdevices which enable the state of the vacuum to be determined. Theinvention goes a step further, however, because when there is a poorvacuum in the tube the anode circuit is automatically interrupted.

A constructional example of the invention is illustrateddiagrammatically in the accompanying drawing. The problem is to protectthe electrodes of the demountable transmitting tube a against the eiectsof a poor vacuum. The tube a is directly congnected to a high vacuumpump b, in this case a molecular pump, which is driven by the motor c.In the suction conduit between the pump b and the tube a an ionizationmanometer d is fitted in a vacuum tight manner, this being shown insection and consisting mainly of a glow discharge tube with itsdischarge gap located in the eld of a permanent magnet. The electrodesof this tube d are connected to a constant alternating voltage e ofabout 2000 volts and the glow current varies with the vacuum. It can -beread off the measuring instrument f and ows through the potentiometerresistance g.

The voltage drop produced by the glow current and depending on thevacuum is picked up at h and passed as a negative grid voltage to thegrid of the amplifying tube i. The poorer the vacuum the higher the gridvoltage and the smaller the anode current will be, until this latterbecomes so small that the electromagnetic relay m which is supplied fromthe network 7c drops out. This relay is located in the remote controlcircuit (not 2 shown in full in the drawing) of the anode circuit of thedemountable tube a which is to be protected and causes its main switchto open.

The same device can also be employed to switch in the demountable tubeautomatically when the vacuum improves. Dne to the difference betweenthe pick-up current and the drop-out current of the relay it is possibleto arrange for the voltage of the main tube to be switched in again onlywhen the vacuum is considerably higher than it is at the instant whenthe relay responds.

If the vacuum is very poor the hot cathode may even burn out. In orderto prevent this a second relay n can be provided whose excitationvoltage is tapped from a smaller section o of the resistance g over thetube p. When the pressure in tube a rises above a certain value which ishigher 'than that at which the first relay m responds, relay n alsodrops out and interrupts with an adjustable time lag the hea.tingcurrentcircuit of the cathode. This delay is expedient so that when a suddendeterioration of the vacuum occurs, such as may occur for instance whena demountable tube is being evacuated, the heating current is notunnecessarily interrupted. The same ionisation manometer d, which servesboth for measuring the high vacum and for interrupting the anode voltagewhen a slight deterioration of the vacuum occurs, can be used to controlthis second and less sensitive relay. The relays m and u preferably haveback contacts for closing circuits of signal or alarm devices, asindicated by the legend Alarm circuits, when the relays drop out to openthe energizing circuits of the tube a.

I claim:

1. In a tube assembly in which the vacuum in the tube is to bemaintained, the combination with a tube having anode and heater circuitsassociated therewith, and a vacuum pump for evacuating the tube, ofrelays having contacts for completing said anode and heater ciruuits,and circuit means responsive to the vacuum within said tube forselectively controlling said relays to open said anode circuit upon anincrease of the tube pressure to one value and to open the heatercircuit upon an increase of the tube pressure to a higher value.

2. In a tube assembly, the invention as recited in claim 1, wherein saidrelays are electromagnetic relays, and said circuit means includes avacuum turbe relay for energizing each of said electromagnetic relays.

3. In a tube assembly, the invention as recited in claim 1, wherein saidcircuit means comprises a. control circuit including a source of currentin series with a tapped resistance and with a circuit element subject tothe vacuum within said tube and having an eective resistance that varieswith the vacuum condition, and energizing circuits for the respectiverelays connected across diierent magnitude portions of said tappedresistance.

4. In a tube assembly in which the vacuum in the tube is to bemaintained, the combination with a high Vacuum transmitting tube havingan anode circuit associated therewith and a vacuum pump for evacuatingsaid tube during operation thereof, of means responsive to a dangerouspressure rise within the tube for opening said anode circuit, said meanscomprising an ionization manometer subject to pressure within said tube,said manometer being a glow discharge tube having spaced electrodes anda permanent magnet establishing a magnetic eld in the discharge gapbetween said spaced electrodes, a resistance connected in series withsaid manometer and therewith across a current source of a magnitudeestablishing a glow current across the discharge gap, an amplifying tubeincluding a grid and an anode cooperating with a cathode, circuitelements connecting said grid and cathode to spaced points on saidresistance in such sense that a negative bias is impressed upon the gridby current through said resistance, whereby the anode current of saidampliertube decreases with an increase in the pressure within saidtransmitter tube, and a relay in the anode circuit of the amplifier tubehaving contacts for completing said anode circuit of the transmittingtube during energization of said relay.

MAX GRAF.

